During recent times, the communication technology has changed from analog type systems to systems which communicate in a digital format. In the telecommunications industry, digital private branch and computer branch exchanges are rapidly replacing analog systems. With the introduction of digital telephones, it was common to rewire buildings which housed the analog system being replaced in order to insure the proper operation of the digital telephones. The cost associated with the rewiring oftentimes was twenty-five to fifty percent of the cost of a digital telephone system and made the transition from an analog telephone system to a digital telephone economically less cost effective. Although some digital telephone systems are designed to operate on standard twisted pair cable, there are restrictions which prevent the telephone system form working on all wires which support an analog system.
Presently, there is automatic test equipment available which is designed to test communication lines for use with analog telephones. The test equipment calculates any or all of the following: impedance, attenuation, cable length and discontinuities. However, these parameters are calculated at frequencies which are applicable only to analog telephone systems and does not facilitate the location of all types of discontinuites.
Time domain reflectometers (TDRs), which are either metallic or optical, have been used in automated cable testers. There is a major difference between optical and metallic TDRs in that optical TDRs only measure attentuation and cable discontinuities of fiber optic cable. A cable parameter such as impedance, which is important in determining the quality of metallic cables, is not applicable to determining the quality of a fiber optic cable. Thus, an optical TDR cannot be used to determine the quality of metallic cables.
In the computer industry, telephone or twisted pair cable has been tested to determine its use in local area network (LAN) applications. Data tranmission rates on LANs can be as high as 10 Mbps. However, this testing has occurred only in a laboratory environment and involves using a network analyzer as well as having access to both ends of the cable.
Generally, existing cable testers test cables within a 300-3400 Hz frequency range. Data is transmitted within a digital system as a frequency of 256 KHz. Thus, existing cable testers are not able to test existing cable for digital applications. Moreover, the existing cable testers require access to both ends of the cable which requires additional personnel and increases cost associated with determining the quality of the communication cable.
Consequently, what is needed is a test apparatus for testing cable for digital applications from one end of the cable.